H/D exchange at sp(3) carbons in the coordination sphere of platinum(II).

The [PtCl(η(1)-CH2CH2OR)(Me2phen)], Me2phen = 2,9-dimethyl-1,10-phenanthroline, complex is indefinitely stable in the solid state; however, when dissolved in protic deuterated solvents at basic pH, it undergoes H-D exchange at the Me2phen Me's. An analogous H-D exchange process takes place in the related [PtCl2(Me2phen)] complex which is sterically strained and very easily can detach one nitrogen of Me2phen yielding a T-shaped species. In contrast, the H-D exchange is considerably slower in the [Pt(OR)2(Me2phen)] complex characterized by smaller size and lower trans-labilizing effect of the oxygen donor ligands. It is suggested that the formation of a T-shaped intermediate could foster the C-H activation via oxidative addition to the metal centre. In accord with this hypothesis, the H/D exchange was found to be considerably slower in analogous complexes with 6,6'-dimethyl-2,2'-bipyridyl (Me2bpy), where the greater flexibility of Me2bpy, as compared to Me2phen, reduces the strain in the four coordinate substrate and hence the propensity to dissociate one end of the bidentate ligand.

Photo-isomerisation of alkenyl complexes of platinum(II): structural, spectroscopic, kinetic and computational investigations.

In this work UVA and blue light have been used to study photo-isomerisation about the C=C double bond in complexes of the type [PtCl(-CH=CHAr)(tmeda)] [Ar = C6H5, (E)-2a; 4-CH3O-C6H4, (E)-2b; 3-NO2-C6H4, (E)-2c; and 3-CH3O-C6H4, (E)-2d]. The progress of the reaction has been monitored by NMR spectroscopy following irradiation of the NMR sample. The NMR data have been complemented with X-ray diffractometric analysis of compounds (E)-2a-c and (Z)-2a. The kinetic data clearly indicate that a monomolecular mechanism is operating with the energy of the irradiating light influencing the rate of isomerisation but not the equilibrium composition, which is only slightly in favour of the Z isomer. DFT and TD-DFT theoretical investigations have been carried out to elucidate the nature of the main electronic transitions in the UV-Vis region and the mechanism of the photo-isomerisation reaction appears to proceed through a C=C bond twist process similar to that involved in purely organic molecules such as stilbene. In the Z isomer, one ortho proton of the phenyl group can come close to platinum (Pt···H(ortho) distance of 2.632 Å in (Z)-2a). In the case of 2c, the difference in chemical shift between the two ortho protons varies from 3.30 ppm in the Z isomer, where interaction with Pt is possible, to 0.60 ppm in the E isomer, where such interaction cannot take place. The analysis of the DFT orbitals indicates that the most shifted H(ortho) is that with a greater positive charge, pointing to an H-bond type of interaction.

Modulation of properties in analogues of Zeise's anion on changing the ligand trans to ethene. X-ray crystal structures of trans-[PtCl2(OH)(η2-C2H4)]- and trans-[PtCl2(η1-CH2NO2)(η2-C2H4)]-.

To get further insight in the reaction of nucleophilic substitution upon changing the ligand trans to a η(2)-olefin, the reactivity of some monoanionic platinum(II) complexes (trans-[PtCl(2)X(η(2)-C(2)H(4))](-), X = Cl(-), 1, OH(-), 2, and CH(2)NO(2)(-), 3) towards pyridines with different steric hindrance (py, 4-Mepy, and 2,6-Me(2)py) has been tested. All crystallographic (2 and 3 reported for the first time) and spectroscopic data are in accord with a platinum-olefin interaction decreasing in the order 2 > 1 > 3, paralleling the decreasing electronegativity of the donor atom (O > Cl > C). Not only the platinum-olefin bond but also the bond between platinum and the ligand trans to the olefin appear to be strongest in 2 (Pt-O distance at the lower limit for this type of bond). In the reaction with py, the ligand trans to the olefin is displaced in 1 and 2. Moreover the reaction is in equilibrium in the case of sterically hindered 2,6-Me(2)py, the equilibrium being shifted moderately or prevalently toward the reagents in the case of 1 and 2, respectively. In the case of 3, the reaction with pyridines leads to substitution of the olefin instead of the carbanion. This is in accord with the observation that carbanions strongly weaken the trans Pt-olefin bond.

New chemistry of olefin complexes of platinum(II) unravelled by basic conditions: synthesis and properties of elusive cationic species.

The evolution in basic medium ([RO-] = 1 M in methanol, R = H or Me) of five-coordinate platinum(II) compounds, [PtCl2(eta2-C2H4)(N-N)], 2a-c, (N-N = N,N,N',N'-tetramethyl-1,2-ethanediamine, a; 2,2'-bipyridyl, b; 1,10-phenanthroline, c) leads to the formation of [PtCl(eta1-CH2CH2-OCH3)(N-N)], 5a-c. The analogous compound 5d (N-N = 2,9-dimethyl-1,10-phenanthroline, d) can also be prepared, but not via transformation of the five-coordinate species 2d in basic medium where it is quite stable. 5d can instead be prepared by reaction of d with a strongly basic methanol solution of Zeise's anion [PtCl3(eta2-C2H4)](-), 1. In such a medium the di-anionic trans-[PtCl2(OR)(eta1-CH2CH2-OCH3)](2-) species (1") reacts with to form exclusively 5d. Hydrolysis of with acids bearing weakly coordinating anions leads to [PtCl(eta2-C2H4)(N-N)]+, 3a-c, as stable cations; upon the same treatment 5d does not generate 3d, but it reacts with HCl to give 2d in almost quantitative yield. Cationic complexes 3b, 3c, here reported for the first time, were reacted with some nucleophiles and their behaviour compared with that of the already known 3a. In 3b, 3c the metal centre competes with the coordinated ethene for binding to nucleophiles; therefore the acetylacetonate anion can either add to the olefin (affording compounds 6b, 6c ) or to the metal ion replacing the ethene ligand (yielding compounds 7b, 7c). Under similar conditions, 3a gives exclusively 6a. Secondary amines readily add to ethene in 3b, 3c, affording the addition products 8b, 8c, which undergo a ready cyclization to an azaplatinacyclobutane ring (9b, 9c). The remarkable ease of the four-membered ring formation has been related to the high electrophilic character of the metal core in 3b, 3c.

The unexpected reactivity of Zeise's anion in strong basic medium discloses new substitution patterns at the platinum centre.

Zeise's anion in strongly basic hydroxylated solvents undergoes unprecedented nucleophilic addition of OR- (R = H, Me, Et) to the eta2-ethene giving trans-[PtCl2(eta1-C2H4OR)(OR)]2- which readily reacts with bidentate nitrogen donors N-N to give Cl- and OR- substitution and formation of [PtCl(CH2CH2OR)(N-N)]. Protonolysis of this stable organometallic species offers a versatile route to cationic [PtCl(eta2-C2H4)(N-N)]+ complexes.

Role of metal ions and hydrogen bond acceptors in the tautomeric equilibrium of nitro-9[(alkylamino)amino]-acridine drugs.

3-nitro-9-[2-(dialkylamino)ethyl)]aminoacridines (alkyl = methyl or ethyl) have been used as ligands towards platinum(If). The end product is a complex in which the acridine acts as a tridentate ligand contributing the two exocyclic nitrogen atoms and one of the two peri carbons. The metallation takes place predominantly at the peri position of the unsubstituted ring. The coordinated acridine is in the imino tautomeric form although, in the free state, it occurs exclusively in the amino form (both in the solid state and in solution). The imino tautomer is considered to be the biologically active form. In the platinated species the N(10)H of the acridine can be involved in strong hydrogen bonding with a chloride ion leading to formation of an association complex, the formation constant has been found to be 1.4+/-10(3) M(-1). The N(10)H...CI interaction can influence the tautomeric equilibrium of the acridine dye also in the uncoordinated species, however, the shift in favor of the imino tautomer is not complete.

Coordination and peri-Carbon Metalation of 1-Nitro-9-[(2-aminoethyl)amino]acridines toward Platinum(II). Evidences for Hydrogen Bonding between Endocyclic N(10)H and Chloride Ion.

The antitumor drugs 1-nitro-9-[(2-(dialkylamino)ethyl)amino]acridines (alkyl = Me, A(1); Et, A(2)) with platinum(II) give tridentate coordination compounds in which the two nitrogens of the ethylenediamine side chain and the C(8) carbon atom of the acridine ring system act as donor atoms. An excess of triphenylphosphine displaces the residual chloride coordinated to platinum but leaves unaltered the tridentate acridine ligand. The structures of [Pt(A(1)-H)Cl], 1, and [Pt(A(1)-H)(PPh(3))]Cl, 3, have been solved by single-crystal X-ray diffraction. 1.CHCl(3) crystallizes in the orthorhombic system, space group P2(1)2(1)2(1) (no. 19), with a = 8.715(1) Å, b = 11.045(2) Å, c = 22.609(4) Å, Z = 4, R = 0.0559, and R(w) = 0.0561 for 1502 reflections with F > 3sigma(F). 3.(1)/(2)CH(2)Cl(2) crystallizes in the monoclinic system, space group P2(1)/c (no. 14), with a = 13.418(3) Å, b = 14.053(3) Å, c = 18.918(4) Å, beta = 97.21(3) degrees, Z = 4, R = 0.0591, and R(w) = 0.0611 for 3608 reflections with F > 4sigma(F). In both complexes the acridine ligand adopts an imino-type configuration with the proton of the exocyclic 9-amino group shifted on N(10). Because of a severe steric interaction between the nitro group in the 1-position and the chelate diamine chain in the 9-position, the acridine moiety is folded about the C(9)-N(10) vector with an average angle between outer rings of 12 degrees. Moreover, the acridine aromatic moiety and the platinum coordination planes are twisted, forming a dihedral angle of ca. 20 degrees. The steric repulsion between the nitro and the diamine groups appears to provide the driving force to metalation. The H(10) proton has a great tendency to be engaged in H-bonding as shown by X-ray and solution studies. The formation of a H-bond with a rather poor acceptor such as the chloride ion can cause a downfield shift of the H-resonance as large as 6 ppm.